Charging photoconductive membranes

ABSTRACT

A method of charging a photoconductive membrane which comprises applying to the membrane a corona-producing voltage at the same time as a gas stream is directed onto the photoconductive membrane in counterflow to charged particles tending to be rejected from the charged area.

' United States Patent 1 [111 3,866,096

Metcalfe et al. 5] Feb..11, 1975 CHARGING PHOTOCONDUCTIVE [58] Field of Search 317/262 A, 3, 4

MEMBRANES {75] Inventors: Kenneth A. Metc alfe, Lockleys; [56] References and Alwin S'. Clements, Largs Bay, both UNITED STATES PATENTS Of Australia 3,555,378 1/1971 Smith et al 317/262 A Ass gnee: The Commonwealth of Australia Soma et al, Care of The Secretary Department of Supply, Parkes, Canberra, Australia Pr'mary Examiner-L [21] Appl. No.: 401,105 A method of charging a photoconductive membrane which comprises applying to the membrane a corona- 30 F A P D producing voltage at the same time as a gas stream is pp Ica nomy ata directed onto the photoconductive membrane in Sept. 8,19 Australia 0628/72 counterflow to charged Particles tending to be U S Cl 317/262 A jected from the charged area. 511, int. Cl .J G03g 15/02 6 Claims, 2 Drawing Figures FIELD OF INVENTION This invention relates to charging photoconductive surfaces or the like for image development and other applications.

BACKGROUND OF INVENTION In the charging of photographic membranes, certain problems have existed in the past which the Applicants have attempted to overcome, and as a result of their research into these problems with the charging of photoconductive surfaces, three Australian Letters Patents have been granted on progressive work, including U.S. Pat. No. 412,769 dated the Aug. 31st, 1965 which taught basically the form of discharge taking place from an electrode angled to a photoconductive surface and illustrating that certain bands remote from the central point of the corona are more suited to the charging of the photoconductive surfaces, for the reason that what generally has been termed as softer charge resulted in which defects were not present which occured where the whole of the corona was used for charging.

A second of the above Australian letters Patent is Pat. No. 412,176 dated the May th, 1967 which taught the art of using the principle of the first patent and utilized a rotating platen with the charging point offset from the platen so that a workpiece on the platen received the uniform charge due to the relative distance of various parts of the workpiece from the charging point, this teaching a practical method of using the particular band of the corona which gave the best charging effects.

The third Australian Patent was Pat. No, 421,643 dated the Nov. 1 1th, 1968 related to a still further improvement in the charging technique in that the corona was again offset from the area being charged to make use of the particular part of the field found to be more desirable. However, to enable the workpiece to be passed linearly through the corona zone, masking members were used which were of a shape such that a uniform charge resulted over the entire workpiece spite of the fact that the point was centrally positioned but shielded to again give a soft charge.

The systems outlined in the prior-art have enabled a better charging technique to be developed in which unwanted parts of the corona are not used in charging'and only the particular bands most suitable are utilized.

Further research in this field has shown however that there is a further problem in charging which appears not to have been previously appreciatedand that is that during charging, there is a reluctance by the photoconductive surface to accept the charges in that the gas ions tend to be repelled from the surface and. Therefore, while a substantial stream of gas ions can be directed onto the surface, a major portion of this stream is not retained by the surface and this appears to be particularly true with the charges which are present in the softer charging areas of the corona.

To understand this, it must be realized, that a corona effect is caused by free electrons ionizing the gas molecules by collision in the substantial field surrounding an electrode. As each ionization releases an additional electron, this in turn effects further ionization to cause what has been termed an avalanche. As the ions stream toward the other electrode a space charge is built up which tends to repel the ions, and this results in both a lower charge and also irregularities.

The effect of this is that while there may be a considerable charging stream directed to the area, the retention of the charge on the surface is only a fraction of this. As it has been found that this loss is substantial in the area of the soft charging bands of the corona, it can be assumed that if a greater proportion of these charges could be retained on the surface, or the surface effect enhanced, then possibly the whole of the corona would be more useable by retaining a bigger proportion of the soft charge which could swamp the central ion effect, or alternatively a higher charge could be achieved on a unit area for a given ionization.

SUMMARY OF INVENTION An object of the present invention therefore is to provide a method of charging in which charge level is increased and loss of charges by rejection is reduced.

We have now found that the charges issuing onto a surface from a corona can be increased or held in place by a gas stream directed onto the surface counter to the direction at which the charges tend to leave the surface. v Thus, for instance, in the case of the invention as described with reference to the three earlier patents it should be assumed that the charge carrying media of the corona strikes the surface at a particular angle or are rejected or deflected by the space charge or gas layer which forms and the ions leave the surface at an angle substantially opposite to the angle at which they approach or strike the surface, althoughthe path is necessarily influenced to some extent by the actual field through which the ions pass. 1

We have found thus that where, for instance a charging point for a corona is directed at an angle to a surface which is to-be charged and is offset from the center, the corona striking the'surfac e, or being deflected by the space charge at the surface, particularly the soft charge areas of the corona, can be retained more effectively on the surface if a gas stream is directed onto the surface at an oppositeangle to ions and gas molecules which tend to bounce from or are repelled from the surface so that the ions and gas molecules collide, the gas stream thus-tending to hold the charged particles at the surface and also to give an agitation which appears to greatly enhance results.

Tests have shown if this is done, that there is a very much greater charge retention and also there is a much more uniform charge of higher density on, the workpiece after such a method has been used on it. i

It is conceivable that the heavier ions from a corona move in a different manner to the more mobile particles and we have demonstrated that the heavier ions generally tend to follow a shorter path from the corona point to the base electrode which is associated with it. This was the basis of Australian Patent No. 412,176 where, using the rotatingbase platen, the heavier ions passed from the corona to the platen near its edgebut the softer charging particles extended further towards the center of the platen largely due to rejection effects by similarly charged particles at the central core. While there was a substantially heavier soft charge near the outer edge of theplaten because of the positioning of the corona, the effect of the charge uniformity was countered by the rotation of the platen so that areas near the perimeter passed the corona point relatively 3 quickly but the central areas of the platen were at all times in the weaker field of the corona.

' Because from our research it would appear that the soft charge areas are more prone to rejection from the photoconductive surface it follows that the use of the gas stream directed to counter this non-retention .ef-

fect,'insures that the soft charge particles are retained in'very much greater number, and it is thus conceivable also'that if this is so, the whole of the corona may be useable provided the loss of particles from the surface is countered by appropriately directed gas streams.

It is certain that the counter flow of the gas supply to the area being charged, as well as having a counter motion effect, induces further ion formation at this area due to the presence of the .ion cloud or charge at the surface of the photoconductor which is located on the second electrode. It is advantageous to use a pure monatomic gas to aid any further ionization effects by lowering the voltage necessary for electron release and consequent ionization, but air has also been found to be effective in insuring charge retention or secondary ionization.

It is known that a corona can be deflected by a gas stream striking it at an angle, but this effect is different from the use of a gas stream to prevent the corona charging medium from being rejected by the gas layer or charge effect at the photoconductive surface. Ac-' cording to this invention, because there is a permeation of the gas at the interfacial layer of the photoconductor, there is physical retention of the gas ions at the photoconductor surface, and also secondary gas ion formation due to the separate gas stream directed to the surface which is to be charged, particularly if the gas stream is one capable of ready ionization and is of the correct type to give the polarity effects required for the particular purpose.

.The gas stream can be directed on to the nip of a biasing roller with similar effects and is found to minimise defects caused by a hard" charge, and it is particularly useful when using .a shielded wire charge system with a pair of gas streams directed from nozzles which are co-extensive with the charging wire and directed generally in the same direction as the ion stream.

BRIEF DESCRIPTION OF DRAWING DETAILED DESCRIPTION The following is an example of how the invention can I be practiced.

' A discharge point positioned to face downwards at an angleto the surface to.be charged and adjacent one edge of same is supplied with a direct current voltage of 10,000 volts negative applied between the point and a platen electrode on which the photoconductor medium is supported, spaced at 8 inches from the platen.

A stream of oxygen or nitrogen gas or, carbon dioxide, or even air, is directed downward at an angle, to-

ward the area being charged, from the opposite edge of v the platen, the gas flow having a velocity of 20 centimeters per second and issuing from a flat'nozzle.

It was found that, under such circumstances, the

value of the charge held on the surface when the gas stream was present was increased by about per cent and the charge was distributed more uniformly.

In the drawing, FIG. 1 illustrates this principle, the platen being designated at 1, the membrane which is being charged at 2, the charging point at 3, the high voltage supply at 4 and the gas-discharge nozzle at 5.

FIG. 2 shows the principle applied to a charging wire, the base electrode being designated at 6, the medium being charged at 7, the charging wire at 8, the directing shield at 9, thehigh voltage generator at 10, and the gas nozzles at 11 and 12. These nozzles are elongatedto be co-extensive with the wire and shield. The nozzles can form part of the shield if required.

We claim:

1. A method of charging photoconductive membranes which comprise applying a corona-producing voltage between a corona discharge electrode and a base electrode, interposing near the base electrode-a photoconductive membrane to be charged, and'simulbon dioxide.

4. The method o f claim 1 wherein the corona discharge is angularly directed to an area to becharged and the said gas is directed to the said .area at a similar angle but from the opposite direction.

5. The method of claim 1 wherein the corona is directed to the surface to be charged from a wire displaced from the said surface but parallel to the said surface, and the gas. is directed to the said surface generally in the direction of movement of the corona ions flowing toward the area-being charged, the said gas being discharged from nozzles which are co-extensive' with the wire and disposed on each side thereof.

6. The method of charging photoconductive membranes according to claim 1 .wherein the said memgas flow urging the charge on to the said surface. I 

1. A method of charging photoconductive membranes which comprise applying a corona-producing voltage between a corona discharge electrode and a base electrode, interposing near the base electrode a photoconductive membrane to be charged, and simultaneously directing a gas stream onto the photoconductive membrane in counterflow to charged particles tending to be rejected from the charged area, and also to perMeate the gas layer at the interface, whereby there is a greater charge retention and secondary ion formation at the surface being charged.
 2. The method of claim 1 wherein the said gas is a monatomic gas.
 3. The method of claim 1 wherein the gas is taken from the group comprising oxygen, nitrogen gas or carbon dioxide.
 4. The method of claim 1 wherein the corona discharge is angularly directed to an area to be charged and the said gas is directed to the said area at a similar angle but from the opposite direction.
 5. The method of claim 1 wherein the corona is directed to the surface to be charged from a wire displaced from the said surface but parallel to the said surface, and the gas is directed to the said surface generally in the direction of movement of the corona ions flowing toward the area being charged, the said gas being discharged from nozzles which are co-extensive with the wire and disposed on each side thereof.
 6. The method of charging photoconductive membranes according to claim 1 wherein the said membrane is displaced from the central core of the corona discharge to receive a soft charge, and the soft charge is retained at the surface of the said membrane by the gas flow urging the charge on to the said surface. 